This invention relates to a channel estimation device for digital telecommunications station, such as a GSM mobile telephone.
In the GSM system each data burst transmitted by a base station includes a xe2x80x9cmid-amblexe2x80x9d including a 26 bit training sequence. These training sequences, are used by the mobile station to calculate channel taps required for demodulation of data within each timeslot transmitted from the CSM base station. The ability of the demodulator to reconstruct error free data is limited by the quality of the channel estimation.
The mobile station in fact uses each training sequence to locate the centre of the signal burst containing it. It does this by comparing the part of the burst where the training sequence is expected to be with a locally generated sequence which matches the expected training sequence. In conventional channel estimation devices, the central sixteen bits of the training sequence are correlated with the expected sequence and the result is used to estimate the timing error so that the channel taps can be set accordingly.
The conventional arrangement operates satisfactorily in the absence of interference, thermal noise and varying Doppler shifts. When these conditions are present, however, the estimation may be sufficiently inaccurate to prevent proper reconstruction and decoding of the data signals.
It is thus an object of the invention to provide a channel estimation device which can provide a more accurate channel estimation in the presence of interference and/or thermal noise than has been possible heretofore.
A channel estimation device in accordance with the invention comprises a first estimator means operating to provide a time error estimate in accordance with the correlation between a central group of bits of a received training sequence with the corresponding bits of an expected training sequence, averaging means for averaging the signal corresponding to each bit of a first sub-group of the bits of the training sequence with the signal corresponding to equivalent bit in a second sub-group of the bits of the training sequence, the first and second sub-groups of bits being expected to be identical, and second estimator means operating to recalculate the timing error estimate in accordance with the correlation between the same central group of bits in the received training sequence corrected by substituting the averaged signals for those contained in the received training sequence and the corresponding bits of the expected training sequence.
In the GSM system, there are eight different training sequences, and the first ten bits of ""each are identical to the last ten. The first and second subgroups of bits could thus be the first ten bits and the last ten bits respectively.
With this arrangement there is found to be an improvement of the raw bit error rate of the order of 0.5 to 1 dB. The amount of extra processing required is small and is thus well suited to incorporation in the DSP (digital signal processor) used for much of the processing in GSM mobile telephones.